TESTING THE SPHERICAL ABERRATION. 157 



that the bluish fog, with lower adjustment, extends further, but 

 with higher adjustment immediately disappears, so that the micro- 

 scopic image appears as a sharply bounded circle of light with 

 greenish-yellow border. Under-correction is characterized by the 

 same contrasts in an opposite sequence. 



(5.) The aberration of the central rays can only be more accu- 

 rately ascertained, if the effect of light thereby occasioned is 

 apparent a case which, according to our experience, hardly ever 

 occurs. 



(6.) If the phenomena mentioned in (4) are undecided (the bluish 

 fog, for instance, remaining perceptible for a short time both with 

 higher and with lower adjustment), then both kinds of aberration 

 are present in the peripheral part of the cone, perhaps in con- 

 sequence of an inaccurate curvature of the refracting surfaces. 



An accurate and practically valuable process of testing has 

 recently been expounded by ^cbbe. 1 It is intended to exhibit the 

 combined action of all the zones of the aperture of the objective in 

 the centre and margin of the field of view, and at the same time 

 to enable the respective images to be clearly distinguished. For 

 this purpose the illumination is regulated by suitable diaphragms 

 having several circular apertures (most conveniently with the 

 apparatus described above on page 110), so that two or three 

 isolated pencils simultaneously traverse the objective in the same 

 number of correspondingly situated zones. For instance, if the 

 objective has an aperture of 6 mm. the track of one of these 

 pencils of light would appear as a bright circular disc of 1 mm. in 

 diameter in the posterior focal plane, near the 

 centre (Fig. 92, middle circle) ; a second track 

 would be situated on the left, from 1 2 mm. from 

 the centre ; the third would be on the right, near 

 the margin. This device shows the sensitive 

 course of the rays, in which all faults of correction 

 are most active. If the Microscope be focused FlG - 92 



on one of the test-plates mentioned on p. 151, in which transparent 

 and opaque lines lie in the same plane, each pencil of light from 

 one of the bands of lines occupying the field of view forms its own 

 image, and the three partial images at a certain point of focal 

 adjustment coalesce and form one colourless and sharply outlined 

 image only when the objective is perfect. In every other case a 

 1 " Archiv fur mik. Anat." Bd. ix. 434. 



